1. Field of the Invention
The present invention relates to a non-regrowth distributed feedback (DFB) semiconductor laser and a method of manufacturing the same and, more particularly, to a method of fabricating diffraction gratings on both sides of a ridge.
2. Description of the Related Art
DFB semiconductor lasers are known as devices which can be used in the fields of optical communication systems such as optical CATVs, pumping light sources for SHG short-wave lasers for high-density information recording or small solid-state lasers, and optical measurement. Conventional distributed feedback semiconductor lasers are formed using two or more steps of epitaxial growth. In a ridge type DFB semiconductor laser formed using two or more steps of epitaxial growth, a grating (diffraction grating) is provided in a laser waveguide layer and thereafter another layer is formed on the waveguide on an epitaxial growth basis.
Recently, in order to avoid the complicated epitaxial growth in two or more steps, the so-called non-regrowth distributed feedback semiconductor lasers have been developed which are fabricated using one step of epitaxial growth, i.e., which does not involve the second epitaxial growth.
For example, in R. D. Martin et al. "CW Performance of an InGaAs-GaAs-AlGaAs Laterally-Coupled Distributed Feedback (LC-DFB) Ridge Laser Diode" IEEE Photonics Technology Letters, Vol. 7, No. 3, pp 244-246, March 1995, an InGaAs-GaAs-AlGaAs distributed feedback semiconductor laser is disclosed in which an active layer and a cladding layer are formed on a substrate by means of epitaxial growth; a ridge stripe is formed on the cladding layer; and a grating is provided on the top portion of the ridge stripe and on flat portions on both sides thereof. In methods of manufacturing such a non-regrowth distributed feedback semiconductor laser, the grating is formed on the entire region of the substrate including the top portion of the ridge type waveguide by means of direct writing with electron beams (EB).
As shown in FIG. 1, however, an unnecessary grating 3 exists in the region of a contact layer on the top portion of a ridge 1 into which a current injected immediately after gratings 2 are formed on flat portions on both sides of the ridge 1. The grating 3 on the top portion of the ridge may obstruct the step of forming an electrode to make connection to the contact layer region. This can result in poor contact and can adversely affect the characteristics of the device.